upgrade from RHEV 3.4 to RHV 4.1

hi,

we are planning to upgrade from RHEV 3.4 to RHV 4.1

in current system, we got RHEV-Manager 3.4 & two hypervisors (RHEL 6.6), and we got many virtual machines running on this cluster

what will be the best way to upgrade to RHV 4.1 ? we want to achieve minimum downtime; all existing virtual machines shall be able to run on the new RHV 4.1 platform